Switching regulators and converters convert DC power from one voltage level to DC power of another voltage level. Switching converters typically utilize a transformer through which an input voltage source is switched to produce an output voltage across the transformer secondary. Depending upon the turns ratio of the transformer, the output DC voltage can be less than or greater than the DC power supplied to the input of the converter. The transformer secondary voltage is rectified and filtered to produce the output DC voltage.
In one family of switching converters, a switching device is driven by a pulse circuit for switching the input DC power through the transformer at a predetermined frequency. The output voltage of the converter can be regulated by a feedback circuit which senses the output voltage of the converter to change the duty cycle of the pulse circuit. In this manner, when more power is required at the converter output, the input switching transistor is driven by a higher duty cycle, thereby delivering more power to the load. Such converters using this principle are known as the pulse-width modulated (PWM) type.
Switching converters of the well-known flyback type include a transistor in series with the transformer primary as a switch which is periodically driven with a pulse to allow current to flow through the transformer primary. Pulse width modulation techniques can also be utilized with flyback regulators to vary the duty cycle or width of the pulse. However, with flyback regulators or converters, the transformer primary functions as an inductor when the transistor switch is turned on. A diode in the transformer secondary prevents current flow therethrough when the primary inductance is being charged with energy. When the transistor switch is turned off, the transformer voltages reverse and the stored energy is discharged through the transformer secondary into the output capacitor and load, thereby replenishing the energy delivered by the capacitor to the load. During this discharge time when the switching transistor is off and the transformer voltages are reversed, the transistor must withstand the reflected secondary voltage in addition to the input supply voltage. For a 1:1 transformer turns ratio, the voltage across the switching transistor can be twice the supply voltage. After all energy has been discharged into the capacitor, the primary switching transistor is again turned on and the cycle repeats. A tertiary voltage sense winding is sometimes added to the transformer to detect when the transformer has not been fully discharged. If, as the result of a fault condition, the sense voltage has not dropped to zero by the beginning of the next cycle, the switching transistor is inhibited from turning on again until the transformer has fully discharged.
Several inherent disadvantages accompany the described flyback switching supply. First, when voltages of large magnitudes are involved, transistors are not available with a breakdown voltage capable of withstanding the reflected secondary voltage added to the supply voltage. As a result, a transistorized flyback switching supply cannot be used in this application.
Another inherent shortcoming of the described type of flyback switching supply is that the voltage sense winding of the transformer is a poor indication of when all the energy has been transferred from the transformer. The voltage from the sensing winding of the transformer is characterized by noise, oscillations and other hash. This makes it difficult to determine when the current in the transformer secondary has been reduced to zero.
From the foregoing, it can be seen that a need exists for a flyback switching supply which is improved with regard to at least two aspects. First, a need exists for a circuit arrangement which prevents voltages in excess of the supply voltage from being impressed across the switching transistor. Secondly, a need exists for a better method of sensing when the current in the transformer secondary reaches a minimum value.